The present invention relates to a non-combustible magnesium alloy having enhanced pyrophoric characteristics and oxidation-resistance.
Up to now, in products for automobiles, such as a cylinder head-cover, oil pan, disc wheel, and transmission case, aluminum alloys with low specific gravity have been extensively used in Korea and abroad.
As a material for further imparting the quality of being light weight, magnesium (density of approximately 1.74 g/cm3) or magnesium alloy, having lower specific gravity than aluminum, has received the spotlight in recent years and is currently being widely used for materials for aircrafts, automobiles, portable machineries, and other daily-used products. In this light, there seems to be a trend of expanding the range of its applications.
In electrochemical terms, however, magnesium has lower electrical potential and is a very reactive metal, which is highly corrosive by means of air, water, and other chemicals it contacts. Furthermore, magnesium has a melting point of 659xc2x0 C., which is similar to that of aluminum, but has a higher vapor pressure and stronger oxidation tendencies, which in turn leads to spontaneous combustion at the temperature of 850xc2x0 C. or higher due to its volatilization. Moreover, unlike aluminum, the MgO layer formed on the surface of the molten metal is not dense, but instead it forms a thick oxidation layer, which in turn leads to defects in casting materials and loss of molten metals. As such, it must be melted in a protective atmosphere by means of using flux and/or inert mixed gas such as CO2+Air+SF6.
However, the above-mentioned flux used during melting and smelting is chloride-based. Hence, in the case of using said flux to prevent oxidation and combustion, there is a problem of significantly lowering the corrosion-resistance of the material by residual chlorides if the processing condition is incomplete.
In order to resolve such disadvantages, rather than using the aforementioned flux, there is a method of melting and casting the same in the atmosphere of mixture of SF6, CO2, and air. The amount of SF6 gas used in order to prevent oxidation in the above method is 0.05% during the stationary state of the molten metal, and depending on the temperature during stirring, 0.01% at 700xc2x0 C., or 1% at the temperature above 700xc2x0 C. The mixed gas used at this point of the process has the effect of changing the characteristics of the Mg oxidation layer while suppressing the continual oxidation and volatilization of the molten metal. Hence, the above mixed gas is currently being used in most of the manufacturing processes. However, approximately 5xcx9c7 tons of CO2, and 0.5 kg of SF6 gas are necessary to produce 1 ton of magnesium. Then, to cast the magnesium alloy, 1 kg of SF6 gas is needed per 1 ton of magnesium. Moreover, CO2 and SF6 gases are major causes of the phenomenon of global warming. Compared to CO2 gas, the harmful effects of SF6 on global warming are substantially more serious. Based on research findings, 1 kg of SF6 has the same effect of global warming as approximately 24 tons of CO2 gas. The resulting effect of producing 1 ton of magnesium is as if approximately 50 tons of CO2 gas was being used. Hence, the destructive effect on global environment, caused by using the mixed gas, is of an enormous scale. Consequently, it is expected that the manufacturing processes of magnesium alloys by means of using the mixed gas will be subject to government regulations in the near future. Moreover, the method of using the mixed gas as above entails the following sets of serious problems: rising costs attributable to the removal of the dross and the usage of the SF6 gas; the burden of environment costs with respect to air pollution; deterioration of workers"" health due to the discharge of toxic gas; deterioration of safety during the mechanical operations (i.e., cutting) due to the extremely low temperature (500xc2x0 C.) of the point of spontaneous combustion, etc.
The objectives of the present invention lie in providing a non-combustible magnesium alloy, which can be used as materials for a wide variety of components, having enhanced oxidation-resistance for melting of the same in air or in the generally non-oxidative atmosphere (SO2, Ar, CO2, N2), and improved pyrophoric characteristics for preventing spontaneous combustion of chips accumulated after the mechanical processing of the components. Further, because it can be melted and casted in an environment that does not include SF6, the present invention brings about beneficial collateral effects of cost reduction, protection of workers"" health, and prevention of environmental pollution, etc.